From 774dd366e901005850f34b5fbcc343879d4a04f0 Mon Sep 17 00:00:00 2001
From: Qyriad <qyriad@qyriad.me>
Date: Sat, 22 Jun 2024 21:22:29 -0600
Subject: [PATCH] trace when the `foo` part of `foo.bar.baz` errors
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Turns errors like:

let
  errpkg = throw "invalid foobar";
in errpkg.meta

error:
       … while calling the 'throw' builtin
         at «string»:2:12:
            1| let
            2|   errpkg = throw "invalid foobar";
             |            ^
            3| in errpkg.meta

       error: invalid foobar

into errors like:

let
  errpkg = throw "invalid foobar";
in errpkg.meta

error:
       … while evaluating 'errpkg' to select 'meta' on it
         at «string»:3:4:
            2|   errpkg = throw "invalid foobar";
            3| in errpkg.meta
             |    ^

       … while calling the 'throw' builtin
         at «string»:2:12:
            1| let
            2|   errpkg = throw "invalid foobar";
             |            ^
            3| in errpkg.meta

       error: invalid foobar

For the low price of one try/catch, you too can have the incorrect line
of code actually show up in the trace!

Change-Id: If8d6200ec1567706669d405c34adcd7e2d2cd29d
---
 src/libexpr/eval.cc                               | 15 ++++++++++++++-
 tests/functional/lang/eval-fail-recursion.err.exp |  6 ++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index a6a64a43c..b01867a09 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1432,7 +1432,20 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     Value * vCurrent = &vFirst;
     // Position for the current attrset Value in this select chain.
     PosIdx posCurrent;
-    e->eval(state, env, vFirst);
+
+    try {
+        e->eval(state, env, vFirst);
+    } catch (Error & e) {
+        assert(this->e != nullptr);
+        state.addErrorTrace(
+            e,
+            getPos(),
+            "while evaluating '%s' to select '%s' on it",
+            ExprPrinter(state, *this->e),
+            showAttrPath(state.symbols, this->attrPath)
+        );
+        throw;
+    }
 
     try {
         auto dts = state.debugRepl
diff --git a/tests/functional/lang/eval-fail-recursion.err.exp b/tests/functional/lang/eval-fail-recursion.err.exp
index 19380dc65..f0057b2d5 100644
--- a/tests/functional/lang/eval-fail-recursion.err.exp
+++ b/tests/functional/lang/eval-fail-recursion.err.exp
@@ -1,4 +1,10 @@
 error:
+       … while evaluating 'a' to select 'foo' on it
+         at /pwd/lang/eval-fail-recursion.nix:1:21:
+            1| let a = {} // a; in a.foo
+             |                     ^
+            2|
+
        … in the right operand of the update (//) operator
          at /pwd/lang/eval-fail-recursion.nix:1:12:
             1| let a = {} // a; in a.foo